They weren't so successful at conquering Europe or China. Of course, one can blame the long supply lines, but in these cases their adversaries were also determined to repulse them.
In contrast, Byzantium and Persia were exhausted from mutual warfare. Tribes related by kinship to Muslim Arabs had served as mercenaries during this period, so there was a lot of tactical knowledge available.
The Byzantines retreated to the highlands of Anatolia upon losing their lower-lying provinces to the Muslims. I am not sure if they launched a determined counter-attack. The Persians did, but they could not fight coherently.
Because non-communist governments wanted to control its spread, and prevent other countries from becoming communist. I guess because communism's style of governance is unfair to some.
Answer:
What replaces steam? What new power source was developed in the late 1800s? In the late 1800s, a new power source—electricity—replaced steam as the dominant source of industrial power.
